There are very few commonly used methods of attaching electronic devices onto circuit boards. The two most common methods used are wave solder and vapor phase/solder reflow processes. Both of these processes have their advantages and can normally be used with packaging styles, such as the Pinless Grid Array (PLGA). The PLGA is a packaging style, which uses coupling feedthrough technology for attaching electronic devices to circuit boards.
Surface attachment of components is gaining favor in this country, because it facilitates greater miniaturization of electronic equipment. There is constant pressure on the industry to reduce the size of electronics within any given device. The utilization of surface attached components will, in almost all instances, reduce the overall size of the circuit.
The cost of printed wiring interconnection is becoming an increasingly larger percentage of the finished value of electronic equipment. Interconnect costs have an effect similar to miniaturization. In an effort to reduce board costs, size reductions are being undertaken, which leads back to the use of surface attached chip components and devices to further aid in reducing size.
Surface attachment techniques eliminate insertion holes, which also reduce board size and eliminate some manufacturing problems. However, through holes or vias are still required in order to interconnect different layers or surfaces. It is quite likely that through hole count will increase, because the potential for internal connections formerly achieved with the component lead is lost in the surface attachment configuration.
The progress toward miniaturization has resulted in the further decrease in the size of through holes. This significant reduction in the size or diameter of the through holes has resulted in the inability to obtain feedthrough solder coupling in a conventional manner.
In the conventional well wave soldering process, electrical components are placed on the top of a board or substrate, and the bottom of the board is contacted with a wave of molten solder. The molten solder flows up through the through holes and establishes a feedthrough connection. However, solder is too viscous to flow through the very small diameter (micro or miniature) holes that are desired in today's electronic board construction. This problem can be overcome in some cases by reducing the flux density of the solder. However, in many cases, the feedthrough diameter is so small that no amount of flux balancing will be effective to establish a flow through the hole.
An example of a prior art coupling feedthrough is illustrated in FIG. 1. A through hole coupling through multiple layers of material is shown, which couples internal and external conductors on two or more of the layers. In the illustrated embodiment for example, a circuit board 12 includes a through hole 14 and a conductive pad 16 on one surface thereof. A layer of solder inhibitor 18 is disposed between the circuit board 12 and an insulator 20, which also has a through bore 22. A single or multiple layer substrate 24 is disposed adjacent insulator layer 20 and may contain one or more internal traces 26 and 28. An insulator layer 30 sandwiches the substrate between it and insulator layer 20. Insulator layer 30 similarly has a through bore 32. A conductive pad 34 may also be on the upper surface of insulator layer 30 and may connect to one or more conductors 36.
In the prior art approach to interconnecting the layers, the through bores 14, 22 and 32 are aligned and are of such diameter that placement of the substrate in contact with a standing wave of solder will result in the solder being pulled, or flow by capillary action, through the through bores, forming a conductive plug of solder 38. The through bores may or may not be pre-metallized as desired. By the prior art technique, the through bores are typically pre-metallized by a thin layer of solder 40 prior to forming a through connection. By the prior art technique, the diameter D of the through holes is sufficient that the solder will flow through the hole and form the continuous conductive plug.
As pointed out above, solder will not flow through the smaller diameter (micro) through holes in this manner.
Accordingly, it is desirable that some means and method be available for solder coupling via miniature through holes.
The applicant has devised an apparatus or system and technique for forming a conductor through these miniature through holes that are too small for normal reflow or capillary action.